Use the link named above.
All your *personal* information will be deleted, as stated on their site.
You can also remove associated accounts (such as facebook etc.). See http://docs.disqus.com/help/73/
URLS:
http://www.disqus.com
http://disqus.com/embed/account/?thread_url=&to_redirect=http%3A%2F%2Fdisqus.com%2Fdashboard%2F%23edit-profile